Vulnerability · Published Aug 20, 2026 · Assigned by GitHub_M
CVE-2026-55586SumatraPDF: Heap out-of-bounds write in vendored CHMLib LZX Huffman table construction reachable from crafted CHM files
Medium 6.6
In plain English
A medium-severity memory corruption in sumatrapdf from sumatrapdfreader. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 7% of all known vulnerabilities).
Official description
SumatraPDF is a multi-format reader for Windows. In 3.6.1 and earlier, a crafted CHM file can supply malformed LZX Huffman code lengths to make_decode_table in ext/CHMLib/lzx.c. In the long-code branch, the function writes new internal nodes through next_symbol before validating that the canonical Huffman table has overflowed. The PRETREE case can write beyond the 104-entry PRETREE_table into adjacent heap state in struct LZXstate when reached through chm_open, chm_retrieve_object, LZXdecompress, and BUILD_TABLE. This produces heap memory corruption in the parser process, while arbitrary code execution has not been demonstrated. No fixed version is available as of this review.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| sumatrapdfreader | sumatrapdf | <= 3.6.1rel |
Weakness: CWE-119, CWE-787 · Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:L